1. Field of the Invention
The invention relates generally to a radar system having an operator-positionable variable range mark. Such a range mark is used by an operator to determine the distance from the radar zero position to the selected target upon which the range mark is positioned. More particularly, the invention relates to such a radar system which additionally employs digital processing of received radar signals.
2. Description of the Prior Art
Previous radar systems which employed a variable range ring operated primarily using analog signal processing in the PPI mode. Received radar signals were displayed at substantially the same rate at which they were received. Such systems worked reasonably well at longer ranges in which the writing rate upon the cathode ray tube screen of the display device of the radar system was sufficiently slow to produce an acceptably high brightness level. Also, for the time periods ordinarily involved in the longer ranges, the range to a target could be determined with a generally sufficient amount of precision. However, for short ranges, the writing rate of the cathode ray tube beam became unacceptably high so that the brightness level was reduced down to unacceptably low levels. Moreover, it became more and more difficult to accurately measure the distance to a target as the range decreased because of the short time periods involved.
In systems employing analog signal processing, the range-mark signal was generated as the output of a timer. The position of the range mark upon the screen of the CRT was determined by the timing constant of an R-C circuit coupled to the timer used to set the time between activation of the timer and pulse output. Most frequently, a potentiometer, used for the resistance, was the operator control used to move the range mark. With this system, a given angle of rotation of the potentiometer moved the range mark on the screen by varying amounts depending on the range scale setting. On the shorter ranges, the range mark moved a relatively large amount for a small potentiometer rotation, while the same rotation would be hardly perceptible on the longest ranges.
In still other systems of the prior art, a storage tube was employed to yield a system having a substantially constant brightness independent of the range scale selected. For these systems and other similar systems, the range mark signal was added to the video signal at the output of the storage tube. When the range scale was changed, the range mark would stay at a constant position from the center of the screen and not move on the screen so as to stay on a target selected on another range.